Servlet Session I: Cookie API

Road Map  Creating Cookies  Cookie Attributes  Reading Cookies  Example 1: Basic Counter  Example 2: Tracking Multiple Cookies  Case Study: Customized Search Engine

The Potential of Cookies Idea Servlet sends a simple name and value to client. Client returns same name and value when it connects to same site (or same domain, depending on cookie settings). Typical Uses of Cookies Identifying a user during an e-commerce session Avoiding username and password Customizing a site Focusing advertising

Cookies and Focused Advertising

Creating Cookies

 Three steps to creating a new cookie: 1)Create a new Cookie Object  Cookie cookie = new Cookie (name, value); 2)Set any cookie attributes  Cookie.setMaxAge (60); 3)Add your cookie to the response object:  Response.addCookie (cookie)  We will examine each of these steps in detail.

Sending Cookies to the Client Create a Cookie object. Call the Cookie constructor with a cookie name and a cookie value, both of which are strings. Cookie c = new Cookie("userID", "a1234"); Set the maximum age. To tell browser to store cookie on disk instead of just in memory, use setMaxAge (argument is in seconds) c.setMaxAge(60*60*24*7); // One week Place the Cookie into the HTTP response Use response.addCookie. If you forget this step, no cookie is sent to the browser! response.addCookie(c);

1. Cookie Constructor  You create a new cookie by calling the Cookie constructor and specifying:  Name  Value  Example:  Cookie cookie = new Cookie (“school”, “NYU”);  Neither the name nor the value should contain whitespace or any of the following characters:  [ ] ( ) =, “ / ;

2. Set Cookie Attributes  Before adding your cookie to the Response object, you can set any of its attributes.  Attributes include:  Name/Value  Domain  Maximum Age  Path  Version

Cookie Name  You rarely call setName() directly, as you specify the name in the cookie constructor.  getName() is useful for reading in cookies. public String getName(); public void setName (String name);

Domain Attributes public String getDomain (); public void setDomain(String domain);  Normally, the browser only returns cookies to the exact same host that sent them.  You can use setDomain() to instruct the browser to send cookies to other hosts within the same domain.

Domain Example  Example: Cookies sent from a servlet at bali.vacations.com would not be forwarded to mexico.vacations.com.  If you do want to the cookie to be accessible to both hosts, set the domain to the highest level:  cookie.setDomain (“.vacations.com”);  Note that you are always required to include at least two dots. Hence, you must specify.vacations.com, not just vacations.com

Cookie Age  In general there are two types of cookies:  Session Cookies: Temporary cookies that expire when the user exits the browser.  Persistent Cookies: Cookies that do not expire when the user exits the browser. These cookies stay around until their expiration date, or the user explicitly deletes them. public int getMaxAge (); public void setMaxAge (int lifetime);

Cookie Expiration  The setMaxAge () method tells the browser how long (in seconds) until the cookie expires.  Possible values:  Negative Value (-1) (default): creates a session cookie that is deleted when the user exits the browser.  0: instructs the browser to delete the cookie.  Positive value: any number of seconds. For example, to create a cookie that lasts for one hour, setMaxAge (3600);

Path  By default, the browser will only return a cookie to URLs in or below the directory that created the cookie. public String getPath(); public void setPath (String path);

Path Example  Example: If you create a cookie at then:  The browser will send the cookie back to  The browser will not send the cookie back to  If you want the cookie to be sent to all pages, set the path to /  Cookie.setPath (“/”);  Very common, widely used practice.

Security  If you set Secure to true, the browser will only return the cookie when connecting over an encrypted connection.  By default, cookies are set to non- secure. public int getSecure (); public void setSecure (boolean);

Comments  Comments: you can specify a cookie comment via the setComment() method. But, comments are only supported in Version 1 cookies.  Hence, no one really uses these methods… public int getComment (); public void Comment (String)

3. Add Cookies to Response  Once you have created your cookie, and set any attributes, you add it to the response object.  By adding it to the response object, your cookie is transmitted back to the browser.  Example: Cookie school = new Cookie (“school”, “NYU”); school.setMaxAge (3600); response.addCookie (school);

Sending Cookies to the Client Create a Cookie object. Call the Cookie constructor with a cookie name and a cookie value, both of which are strings. Cookie c = new Cookie("userID", "a1234"); Set the maximum age. To tell browser to store cookie on disk instead of just in memory, use setMaxAge (argument is in seconds) c.setMaxAge(60*60*24*7); // One week Place the Cookie into the HTTP response Use response.addCookie. If you forget this step, no cookie is sent to the browser! response.addCookie(c);

Reading Cookies

 To create cookies, add them to the response object.  To read incoming cookies, get them from the request object.  HttpServletRequest has a getCookies() method.  Returns an array of cookie objects. This includes all cookies sent by the browser.  Returns a zero-length array if there are no cookies.

getValue/setValue –Gets/sets value associated with cookie. –For new cookies, you supply value to constructor, not to setValue. –For incoming cookie array, you use getName to find the cookie of interest, then call getValue on the result. –If you set the value of an incoming cookie, you still have to send it back out with response.addCookie.

Reading Cookies  Once you have an array of cookies, you can iterate through the array and extract the one(s) you want.  Our next few examples illustrate how this is done.

Example 1: RepeatVisitor.java  This servlet checks for a unique cookie, named “repeatVisitor”.  If the cookie is present, servlet says “Welcome Back”  Otherwise, servlet says “Welcome aboard”.  Example: Listing 8.1

Using Cookies to Detect First-Time Visitors public class RepeatVisitor extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { boolean newbie = true; Cookie[] cookies = request.getCookies(); if (cookies != null) { for(int i=0; i<cookies.length; i++) { Cookie c = cookies[i]; if((c.getName().equals("repeatVisitor"))&& (c.getValue().equals("yes"))) { newbie = false; break; }

Using Cookies to Detect First-Time Visitors (Continued) String title; if (newbie) { Cookie returnVisitorCookie = new Cookie("repeatVisitor", "yes"); returnVisitorCookie.setMaxAge(60*60*24*365); response.addCookie(returnVisitorCookie); title = "Welcome Aboard"; } else { title = "Welcome Back"; } response.setContentType("text/html"); PrintWriter out = response.getWriter(); … // (Output page with above title)

Using Cookies to Detect First-Time Visitors (Results) (run example)run example

Example 2: CookieTest.java  Illustrates the differences between session and persistent cookies.  Creates a total of six cookies:  Three are session cookies  Three are persistent cookies  Servlet also uses request.getCookies() to find all incoming cookies and display them.  Listing 8.2:

Differentiating Session Cookies from Persistent Cookies public class CookieTest extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { for(int i=0; i<3; i++) { Cookie cookie = new Cookie("Session-Cookie-" + i, "Cookie-Value-S" + i); // No maxAge (ie maxAge = -1) response.addCookie(cookie); cookie = new Cookie("Persistent-Cookie-" + i, "Cookie-Value-P" + i); cookie.setMaxAge(3600); response.addCookie(cookie); }

Differentiating Session Cookies from Persistent Cookies (Cont) … // Start an HTML table Cookie[] cookies = request.getCookies(); if (cookies == null) { out.println(" No cookies"); } else { Cookie cookie; for(int i=0; i<cookies.length; i++) { cookie = cookies[i]; out.println (" \n" + " " + cookie.getName() + "\n" + " " + cookie.getValue()); }

Differentiating Session Cookies from Persistent Cookies Result of initial visit to CookieTest servlet Same result as when visiting the servlet, quitting the browser, waiting an hour, and revisiting the servlet.

Differentiating Session Cookies from Persistent Cookies (run)run Result of revisiting CookieTest within an hour of original visit (same browser session) I.e., browser stayed open between the original visit and the visit shown here

Example 3: CookieUtilities Utility class (from coreservlets package): - that simplifies the retrieval of a cookie value, given a cookie name. - if value is not set, it will be set to defaultvalue that you supply to the method.  You can use in all your programs that deals with cookie retrieval  It saves time because you do not have to repeat same steps and details every time you deal with cookie (hide details):  Retrieve all cookies array  go through loop to find name,value.  CookiesUtilities two Main static methods (details 8.3 in book)  getCookieValue (HttpServletRequest request, String cookieName, String defaultValue)  // given request object and cookie name and defaultvalue, getCookieValue method return  // a cookievalue with matching name, in no value found return  //“defaultvalue” supplied  getCookie (HttpServletRequest request, String cookieName)  // given request object and cookie name, getCookie method return a cookie with //matching name, in no name found return “null”… see details for this method (8.3 in book)

CookieUtilities: Finding Cookies with Specified Names public class CookieUtilities { // has two methods (details 8.3 in book) //Given the request, a name, a default value, this // method tries to find the value of the cookie with // the given name. // if no cookie matches the name, the designated // default value is returned public static String getCookieValue (HttpServletRequest request, String cookieName, String defaultValue) { Cookie[] cookies = request.getCookies(); if (cookies != null) { for(int i=0; i<cookies.length; i++){ // loop through available cookie Cookie cookie = cookies[i]; if (cookieName.equals(cookie.getName())) { return(cookie.getValue());// return value for match name } } // if no cookie match, return default value return(defaultValue); } }

Another helpful Utility (coreservlets package): LongLivedCookie (8.4 in book) // small class you can use instead of Cookie if you want to your //cookie to automatically bet set (persists) for one year rather //than be deleted when browser closes(session cookie by default) public class LongLivedCookie extends Cookie { public static final int SECONDS_PER_YEAR = 60*60*24*365; public LongLivedCookie(String name, String value) { super(name, value); setMaxAge(SECONDS_PER_YEAR); }

Applying Utilities: RepeatVisitor2 example (8.4 in BOOK) produces same result as last example (8.2) but using utilities provided by coreservlets package to make it easier (run)run public class RepeatVisitor2 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { boolean newbie = true; // CookieUtilities.getCookieValue loops through available cookie and return // value for cookie name “ repeatVisitor2” which is “yes” // if no match rturn default “no” String value = CookieUtilities.getCookieValue(request, "repeatVisitor2", "no"); if (value.equals("yes")) { newbie = false; } String title; if (newbie) { //if new_cookie, create a new one and set to one year using LongLivedCookie LongLivedCookie returnVisitorCookie = new LongLivedCookie("repeatVisitor2", "yes"); response.addCookie(returnVisitorCookie); title = "Welcome Aboard"; } else { title = "Welcome Back"; }

Modifying Cookie Values Replacing a cookie value Send the same cookie name with a different cookie value. Reusing incoming Cookie objects. Need to call response.addCookie; merely calling setValue is not sufficient. Also need to reapply any relevant cookie attributes by calling setMaxAge, setPath, etc.—cookie attributes are not specified for incoming cookies. Usually not worth the bother, so new Cookie object used To delete cookie: Instructing the browser to delete a cookie Use setMaxAge to assign a maximum age of 0.

Example : ClientAccessCount (8.6 in book) - display the number of hits for each user. - The value of the cookie will be the counter - change the value of cookie with each visit (increment )  A few weeks back, we created a simple Counter servlet that keeps track of the number of “hits”.  Now, we want to display the number of hits for each user.  This is relatively simple to do:  We just create a counter cookie  The value of the cookie will be the counter  Increment the counter with each visit  and increment the counter cookie at each visit.  Listing 8.6:

Tracking User Access Counts public class ClientAccessCounts extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // utility return cookie value(representing counter) as a string // “10”) for cookie name “accessCount”). //If no value, return default “1” String countString = CookieUtilities.getCookieValue(request, "accessCount", "1"); int count = 1; try { // convert string value “1” to integer count = Integer.parseInt(countString); // convert count to integer } catch(NumberFormatException nfe) { } LongLivedCookie c = new LongLivedCookie("accessCount", String.valueOf(count+1)); // increment counter by 1 // add cookie info to to response (with new updated counter as value of cookie) response.addCookie(c);

Tracking User Access Counts (Continued) // print result or number of visits per browser out.println(docType + " \n" + " " + title + " \n" + " " + title + " \n" + " This is visit number " + count + " by this browser. \n"+ " "); }

Tracking User Access Counts (Results) (run live)run live

Summary To create a cookie:  Create a new Cookie Object  Cookie cookie = new Cookie (name, value);  Set any cookie attributes  Cookie.setMaxAge (60);  Add your cookie to the response object:  Response.addCookie (cookie) You can use utilities provided with coreservlets to make process easier  CookieUtilities.getCookieValue  LongLivedCookie

Midterm Exam Midterm Wed MAR 2 (during class) Required readings All PowerPoint lectures posted on the website Core Servlets: Chapter 1, Chapter 2 (skip sections , 2.11) Chapter3 Chapter 4 (skip sections ), Chapter 5 (skip sections 5.4 and 5.6) Chapter 6, Chapter 7 (Skip Sections ) Chapter 8 Chapter 19 Questions: will be based on Core Servlets readings, lectures, examples and Homework

Midterm Exam (format) Questions: will be based or readings, lectures, examples and Homework Multiple choice (testing main concepts) 1 write complete servlet (30 pts) (similar to example and to homework: Generate and parse forms Read and write to file Get data from form, headers and cookies Keep persistent counts Use data structure and other manipulations similar to homework Your servlets will be graded based on syntax and does the code actually work, and comments and modularity Paragraph questions 3- 8 (find errors and why, etc..)